More internal refinements of the ascii splitting algorithm.

More internal refinements of the ascii splitting algorithm.

_encode_chunks(): Pass maxlinelen in instead of always using
self._maxlinelen, so we can adjust for shorter initial lines.
Pass this value through to _max_append().

encode(): Weave maxlinelen through to the _encode_chunks() call.

_split_ascii(): When recursively splitting a line on spaces
(i.e. lower level syntactic split), don't append the whole returned
string.  Instead, split it on linejoiners and extend the lines up to
the last line (for proper packing).  Calculate the linelen based on
the last element in the this list.